Vascular lesions comprising enlarged or ectatic blood vessels have been successfully treated with lasers for many years. In the process, called selective photothermolysis (SP), the lesion is irradiated with laser light. The wavelength or color of the laser light is chosen so that its energy is preferentially absorbed into the lesion, the target tissue. Most commonly in the context of vascular lesions, such as portwine stains for example, hemoglobin of red blood cells within the ectatic blood vessels serves as the chromophore. Ideally, these cells absorb the energy of the laser light and transfer this energy to the surrounding vessels as heat. If this occurs quickly and with enough energy, the surrounding vessels reach a temperature to denature their proteins, which leads to their ultimate destruction. The fluence to reach the denaturation of the vessels is calculated to be that necessary to raise the temperature of the targeted volume within the vessel to about 70xc2x0 C. before a significant portion of the absorbed laser energy can diffuse out of the vessel.
Flash lamp excited dye lasers meet the wavelength constraints required for selectivity. These lasers are readily tunable to generate pulsed laser light in a range around 580 nm. The greatest disparities between the absorption of hemoglobin and melanin, the principle pigment in the skin, exist in this range.
Wavelength aside, the intensity and pulse width of the laser light must also be optimized in order to maximize selectivity. Proper pulse duration and intensity are important to attain temperatures necessary to denature the vessel""s protein without heating too quickly the red blood cells. Boiling and vaporization are desirably avoided since they lead to mechanical, rather than chemical, damage, which can increase injury and hemorrhage in tissue surrounding the lesion. These constraints suggest that the pulse duration should be longer with a correspondingly lower intensity to avoid vaporization. Because of thermal diffusivity, energy from the laser light pulse must be deposited quickly, however, to minimize heat dissipation into the surrounding tissue. The situation becomes more complex if the chromophore is the blood cell hemoglobin within the lesion blood vessels, since the vessels are an order of magnitude larger than the blood cells. Radiation must be added at low intensities so as to not vaporize the small cells, yet long enough to heat the blood vessels by thermal diffusion to the point of denaturation and then terminated before tissue surrounding the blood vessels is damaged.
Theory suggests that the length of the laser light pulse should be on the order of milliseconds, especially for adult patents having characteristically thicker and larger blood vessels. Commercially available dye lasers, however, are generally limited in the pulse durations to approximately 0.5 msec.
A number of attempts have been made to increase the pulse length of dye lasers. One approach is disclosed in U.S. Pat. No. 4,829,262 granted to one of the present inventors. This invention was directed to overcoming thermal distortion in the lasing medium, which leads to loss of the resonating modes. Special resonator optics were proposed that would be less sensitive to opto-acoustic perturbations. Other attempts to increase pulse length have been made by implementing planar waveguide lasers. See Burlmacchi, et al., xe2x80x9cHigh Energy Planar Self Guiding Dye Laser,xe2x80x9d Optics Communication, 11(109) (1974).
Recent research suggests that special resonators do not prolong pulse duration longer than standard resonator designs. This realization leads to the conclusion that there must be another reason for the quenching of the lasing action than thermal distortion. Subsequent studies on long pulse flash lamp excited dye lasers show that it is nearly impossible to extract pulses from a flash lamp excited dye laser more than one millisecond long and still meet the energy requirements of an output greater than one hundred millijoules needed for SP.
It seems that induced absorption could be a factor in quenching the lasing action. Although transient absorption can be induced, the largest contribution is considered to be permanent transformation in the dye to a light absorbing specie. The dye concentration is set for uniform absorption of pump light across the short dimension of the dye cell, approximately 4 mm. The concentration optimizes at about 7xc3x9710xe2x88x925M of dye solution. Meanwhile, the laser length is 600 mm or 150 times longer than the dye cell diameter. A 1/e transmission loss along the gain length would overcome any gain in the laser. The concentration of absorbing specie need only be minuscule, on the order of 3xc3x9710xe2x88x927 M to stop any gain. This small concentration of absorbers can be readily generated during the excitation pulse.
In light of the fact that research seems to establish that a dye laser can not produce the necessary pulse widths, the present invention is based upon the recognition that the required pulse widths could be achieved by implementing multiple dye lasers and time multiplexing their output beams. For example, if the required pulse width is on the order of two msec, the pulse laser beams from two lasers, each being approximately 0.8 msec long could be multiplexed in time and combined to effectively meet this width specification.
Moreover, the implementation of time multiplexed multi-colored pulse laser beams allows the dynamic tracking of the absorption spectra of the chromophore, hemoglobin for example, as it is heated. With temporal multiplexing, lasers of different colors can be used to optimize the selectivity in response to the predicted temperature of the target tissue.
As a result, in general according to one aspect, the invention features a long pulsed laser device for selective photothermolysis. This device comprises at least two pulsed lasers, generating successive laser pulses. The laser can be coordinated by a synchronizer that sequentially triggers the laser. A combining network merges the pulse laser beams into a combined bean and a delivery system conveys the combined laser beam to a patient. Such a combined beam may have an energy of 100 millijoules and a pulse duration from 1 to 10 msec.
In general, according to another aspect, the invention features a method for generating a long effective laser pulse for a selective photothermolysis therapy. This method comprises successively triggering at least two pulse lasers to generate pulsed laser beams. These beams are then combined into a combined beam having all effective pulse width equal to a combination of the pulsed laser beams. Finally, the combined beam is delivered to a patient thought a delivery system.
In general, according to another aspect, the invention features a selective photothermolysis method. This method comprises irradiating a tissue section of a patient with a pulsed laser beam having a changing color across a time period of the pulse. This color is selected to maximize absorption in a target tissue of a patient in response to heating caused by a preceding portion of the pulse.
The above and other features of the invention including various novel details of construction and combinations of parts, and other advantages, will now be more particularly described with reference to the accompanying drawings and pointed out in the claims. It will be understood that the particular method and device embodying the invention is shown by way of illustration and not as a limitation of the invention. The principles and features of this invention may be employed and various and numerous embodiments without the departing from the scope of the invention.